Negative chemical ionization mass spectrometry has been employed for its enhanced ionization efficiency for the detection and quantitation of electron capturing derivatives of volatile molecules. Melatonin has been quantitated in human plasma at the part per trillion level (picogram/ml) using an improved ion source which contains collinating magnets to stabilize high sensitivity measurements and a discharge plasma as an alternate electron source. N-acetylserotonin and serotonin have also been measured with enhanced sensitivity. A collaborative NIH-NBS task force has been initiated to study ion source characteristics and detemine sensitivity limits by various ionization methods. A selected ion recording system operating under computer control has been completed in a collaborative effort with CSL/CCRT. Hardware and software are being evaluated on an LKB 9000.